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Friday, June 23, 2017

Maybe quantum mechanics isn't so weird after all

I more or less believe that on general principle, but I don't quite follow this interesting article by Philip Ball, Quantum common sense. I haven't read it with care, at least not yet. But I wanted to park some quotes here for the record. I thought this was kind of neat:

It’s not enough, though, for a quantum state to survive decoherence in order for us to be able to measure it. Survival means that the state is measurable in principle – but we still have to get at that information to detect the state. So we need to ask how that information becomes available to an experimenter. (Really, who’d have thought there is so much to the mere act of observation?)

Here’s the exciting answer: it’s precisely because a quantum system interacts with its environment that it leaves an imprint on a classical measuring device at all. If we were able, with some amazing instrument, to record the trajectories of all the air molecules bounding off the speck of dust, we could figure out where the speck is without looking at it directly; we could just monitor the imprint it leaves on its environment. And this is, in effect, all we are doing whenever we determine the position, or any other property, of anything: we’re detecting not the object itself, but the effect it creates.

Just as coupling the object to its environment sets decoherence in train, so too it imprints information about the object onto the environment, creating a kind of replica. A measurement of that object then amounts to acquiring this information from the replica.

Later:

What Quantum Darwinism tell us is that, fundamentally, the issue is not really about whether probing physically disturbs what is probed (although that can happen). It is the gathering of information that alters the picture. Through decoherence, the Universe retains selected highlights of the quantum world, and those highlights have exactly the features that we have learnt to expect from the classical world. We come along and sweep up that information – and in the process we destroy it, one copy at a time.

Decoherence doesn’t completely neutralise the puzzle of quantum mechanics. Most importantly, although it shows how the probabilities inherent in the quantum wave function get pared down to classical-like particulars, it does not explain the issue of uniqueness: why, out of the possible outcomes of a measurement that survive decoherence, we see only one of them. Some researchers feel compelled to add this as an extra (you might say ‘super-common-sensical’) axiom: they define reality as quantum theory plus uniqueness.